1. Field of the Invention
The present invention relates to a digital communication system for a NTSC color-television signal, and in particular to a method of detecting and correcting a parity error and to a circuit to work the method.
2. Description of the Related Art
In the communication system for transmitting an analog color-television signal by means of digital transmission, the analog color-television signal is, as is well known, converted to a digital color-television signal, coded and multiplexed in the transmitter in order to transmit the coded digital color-television signal by means of time division multiplexing. For time division multiplexing transmission, it is usual to take a horizontal scanning period as a transmission frame. Hereafter a digital color-television signal will be referred to as a digital signal. The transmitted coded digital signal is received by the receiver, where the signal is demultiplexed, decoded and converted to an analog color-television signal.
When it is required to detect a transmission error, a first parity is generated from the coded digital signal in the transmitter and inserted into the transmission frame as a parity bit in order to be transmitted along with the coded digital signal. The first parity is separated in the receiver from the coded digital signal at the demultiplexing stage, and collated with a second parity generated from the separated coded digital signal. If a transmission error takes place in the coded digital signal, it causes the second parity to be different from the first parity. In this way, occurrence of a transmission error is judged from the parity check.
In order to correct the transmission error or a parity error occurring on the transmission line, the typical method has been to substitute the digital signal (provided by decoding the coded digital signal) in a horizontal scanning period in which a transmission error has occurred (hereafter referred to as the present horizontal scanning period), with the digital signal in the preceding horizontal scanning period in the same frame period. In order to work this method, a substitute circuit controlled by the parity check signal is provided in the receiver. The substitute circuit has a buffer memory for storing a digital signal in the last at least one horizontal scanning period. When the parity check signal is active, indicating that the second parity is different from the first parity, the substitute circuit, in lieu of the digital signal in the present horizontal scanning period, delivers the digital signal in the preceding (i.e. the last) horizontal scanning period in the same frame period. In this way, the disturbance in a color-television picture due to transmission error is made indistinguishable.
A problem encounterd in the method described above has been that, since a digital signal in error in the present horizontal scanning period is substituted with the digital signal in the preceding horizontal scanning period, when the transmission error lasts for two or more, for example N horizontal scanning periods (N is a positive integer), the error-included digital signal in the present horizontal scanning period will be substituted for an digital signal in error in the next horizontal scanning period, which causes a disturbance in the color-television picture such as a distinct horizontal streak.
This disadvantage may be avoided by substituting a block of the digital signals in N horizontal scanning periods including the horizontal scanning period in which the transmission error occurs, with a block of the digital signals in the preceding N horizontal scanning periods. While by this block-for-block substitution the disturbance is avoided, it causes another problem that the color-television picture becomes unnatural, because a part of the color-television picture is repeated in the following part of the same picture frame.